Hybrid vehicles are vehicles with two or more power sources in a drivetrain. The most common automotive hybrid being a gasoline-electric hybrid, having an internal combustion engine and an electric motor/generator. Typically hybrids use both an engine and a motor to improve fuel consumption, emission, and performance by switching between the two at optimal times when either the engine or the motor may be more efficient than the other, or in a combination of the two when it is more efficient than the use of only one, depending on the hybrid system and vehicle mode.
Hybrid vehicles are generally classified by the division of power between sources; both sources may operate in parallel to simultaneously provide acceleration or they may operate in series with one source exclusively providing the acceleration and the second being used to augment the first's power reserve.
Series hybrids, also referred to as extended-range electric vehicles or range-extended electric vehicles, are designed to be run mostly by a motor, but have a gasoline or diesel generator to recharge the battery. In a series-hybrid system, the engine drives an electric generator instead of directly driving the wheels. The generator provides power for the driving motors. In short, a series-hybrid is driven only by the motor.
Parallel hybrids may be a traditional style in which an engine and motor are coupled at an axis in parallel such that the speeds at this axis are identical, or directly proportional to each other by reduction or offset gearing, so that the supplied torques are added together on the same output shaft. When only one of the two sources is being used, the other must either rotate in an idling manner or be disconnected by a clutch. The two sources may be applied directly to the same shaft, and as such the speeds in this configuration are equal and the torques are cumulative when entering the transmission. The motor adds or subtracts torque to the system depending on whether it is consuming current (driving the vehicle) or producing current (charging a battery).
Parallel hybrids may also be a power-split style, also sometimes referred to as a series-parallel hybrid. The power-split hybrid allows for either source to provide all of the power separately or for the sources to provide varying amounts of power at the same time to provide the needed power at the wheels. This system usually has two generators, one similar to that of the generator connected to the engine in a series system, and the other being that of a traction motor/generator found in a parallel system. The output shafts from the engine and the traction motor may be combined by a series of gearing which does not require a disconnect clutch.
Hybrid vehicles may also incorporate an automatic transmission in the driveline between one or more of the sources and the drive wheels. An automatic transmission, also called an automatic gearbox, can automatically shift to different gear ratios as the vehicle starts to move, freeing the driver from having to shift gears manually. Most automatic transmissions have a defined set of gear ranges, and are referred to as a step-ratio automatic transmission. A fundamentally different type of automatic transmission is the continuously variable transmission or CVT, which can smoothly and steplessly alter its gear ratio by varying the diameter of a pair of belt or chain-linked pulleys, wheels, or cones. Step-ratio automatic transmissions have predefined shift schedules traditionally mapped by using vehicle speed and driver torque demand or throttle position. The shift schedules are designed to allow an engine to run in a band of optimal engine speeds while providing the desired vehicle speed and final torque output.
For traditional parallel hybrids, for example with the motor being disposed between the engine and a transmission, the engine is typically connected to the driveline by an engaged disconnect clutch during hybrid drive. The engine speed is then determined by the current vehicle speed and the current gear of the transmission when the engine engages. The current gear is selected by the shift schedule which is not optimized for the parallel powertrain configuration, especially when considering if the motor is in a traction mode consuming current from a battery or in a charging mode when providing current to the battery. From an energy management point of view, the engine speed and the motor speed are critical factors to determine the overall system efficiency. Thus it is desirable to have the freedom to move the engine speed in order for the system to operate more efficiently.